Rheinberg illumination

An interesting variance of darkfield is Rheinberg illumination, discovered in 1896 in London by Julius Rheinberg. The major difference between darkfield and Rheinberg illumination is color. Whereas in darkfield, the background is black and the subject is white, Rheinberg goes a step further and creates a colored background and a colored subject.

The condenser in a brightfield microscope illuminates the specimen with a solid cone of light. There are some simple additions that can be made to even inexpensive microscopes that dramatically enhance the image by manipulating the way light hits the specimen. These methods are best accomplished using low power objectives (4X to 20X).

Darkfield illumination involves the use of a special darkfield condenser or the use of a patch stop of black paper centered on a round, clear acetate disk placed in the filter holder of an Abbe condenser. The latter, of course, is the cheapest way to achieve darkfield conditions, especially with 4X to 20X objectives. High-power objectives, such as 40X, 60X and 100X oil immersion, really require the use of special darkfield condensers to be fully effective.

The simplest form of Rheinberg illumination involves a central stop which is made of a colored translucent material, the image background becomes the color of that material. By adding other colors to the outer cone, different objects refract those different colors. The condenser diaphragm should be wide open and the unit racked all the way up. But the condenser can be slightly raised or lowered for the best effect.

The colored filter materials are readily available from craft and art stores and can produce impressive effects as illustrated with this image of a diatom. This technique also gives the impression of depth in the image.